Various methods are already known for making castings using patterns or models gasifiable at relatively high temperature. These methods are known under the term full mold casting and have the common feature that the pattern is "devoured" by the melt flowing in, the casting rate conveniently being so adjusted that cavities between the entering melt and the disappearing pattern are avoided as far as possible. The plastic for the lost pattern, usually polystyrene, usually has added thereto a radiation-absorbing dye to prevent gasification of the pattern before introduction of the melt. For exact castings it is generally necessary to carefully compact the molding sand or admix binders.
Although the full mold casting technique has meanwhile established a firm position in casting methods and is at present the most economical method in the production of large individual castings, it has not been able to completely establish itself in series and mass-produced casting. This was in particular because, for the reasons explained below, thin-walled castings could not be made or could not be made with the necessary dimensional accuracy of the wall thicknesses.
The readily gasifiable plastic for disposable patterns is provided in the form of small solid beads coated with a propellant or foaming agent. The introduction of the foaming agent is in the usual manner by the chemical industry. This preliminary product having a density of the order of magnitude of 600 kg/m.sup.3 is then prefoamed under the action of heat to give a granulate consisting of expanded beads or grains and having a density of only 20 kg/m.sup.3 or less. The granulate thus prefoamed is then introduced in likewise usual manner with carrier air into a foam mold until it fills the latter to a good degree of filling, whereafter subsequent foaming in the foam mold is effected usually by introducing steam, in which the individual expanded foamed plastic beads are pressed against each other and against the walls of the mold cavity by another although small volume increase to establish the necessary mutual connection and thus generate the foamed plastic body with the contour of the foam mold. In the foam mold either foam blocks can be produced, which in various working steps such as cutting and/or material-removing operations possibly with adhesion of the individual elements thus formed are processed to give the pattern, or alternatively in particular with relatively small castings the contour of the foam mold is adapted to the contour of the entire casting or a region thereof from the start so that in the foam mold a finished pattern or a finished pattern part is produced directly. For relatively small castings of complicated shape the latter procedure is of particular importance in economic production of a large number of disposable patterns of complicated shape for series production of castings.
The expanded beads of the prefoamed granulate introduced into the foam mold have with regard to the degree of prefoaming and the bead size of the preliminary product usually employed for the present purpose an average diameter of about 2 mm. With a wall thickness to be formed at the casting pattern and correspondingly a cavity in the foam mold having a width of for example 5 mm only two expanded beads can thus enter the narrow cross-section of the cavity without the expanded beads pressing each other against the wall of the mold. If a third bead drops into the gap between two such adjacent beads it tends to push the two outer beads apart and thus expands them against the wall of the foam mold. The friction between the expanded beads and the wall of the mold is relatively high so that such pressing of expanded beads against said wall can greatly obstruct or completely prevent movement of the beads. Thus, even when the filling operation is supported by carrier air, partial vacuum or the like, it is impossible to be sure that even the thin cross-section will be completely homogeneously filled with expanded beads. In particular under unfavourable conditions, for example when comparatively large expanded beads happen to meet, flaws can thus easily arise which are not filled up to the maximum density with expanded beads.
In particular with patterns with such thin walls in order to permit a troublefree and damage-free manipulation the pattern must be coated with a solidifying covering; such a coating of refractory, for example ceramic, material, frequently misleadingly referred to as blackwash, is also necessary when in the casting mold a forming material is used which is not adequately temperature-resistant with respect to the melt and consequently must be protected from direct contact therewith, for example quartz sand or steel granulate in the production of steel castings. When applied to the surface of the pattern the coating material penetrates into the cavities on the surface side and into outwardly open gussets between adjacent beads pressed against each other and consequently forms an irregular surface on the casting formed in the place of the foamed pattern. If there are additionally on the pattern surface flaws due to incomplete filling with expanded beads, the coating material can penetrate relatively far behind the theoretical surface line of the foamed plastic pattern. During casting the foamed material of the pattern gasifies whereas the corresponding contour of the coating material formed at the previous pattern surface remains. Thus, any penetration of the coating material into the surface of the foamed pattern remains behind as reduction in cross-section on the casting. In the region of a thin wall of the casting such a cross-section reduction in the foamed pattern can result in failure to obtain the necessary strength or other dimensional requirements and waste or, if the cross-sectional weakening is not recognized in good time, subsequent damage due to breakage. To recognize patterns not suitable for casting the pattern surface must be exactly investigated and this is not possible after application of the coating material because when this has been done only the outer surface of the coating material which does not come into contact with the melt is visible.
For this reason it is essential for surfaces of high quality and dimensional accuracy to be produced on the casting for the coating material for the foamed material pattern to be sure of finding a smooth surface of the foamed material as far as possible completely free of pores, let alone faults. Such smooth surfaces are of decisive importance particularly in the region of particularly small wall thicknesses to obtain in the casting the thin wall with homogeneous thickness and without weak points. However, precisely with small wall thicknesses beneath about 7 mm, macroscopic pores occur due to incomplete filling of the cross-section with expanded beads, as explained above, which lead to corresponding pores in the casting, whereas with smaller wall thicknesses still of only a few millimeters in spite of precautionary measures no practicable filling can be achieved at all so that series production is impossible even when the quality standards are low.
It has been known in the packing industry for example for a long time for the production of egg containers, cups, etc, to produce small wall thicknesses down to the order of magnitude of 1 mm. However, the production of particularly thin wall thicknesses is achieved simply by reducing the degree of the preliminary foaming or preexpanding. For a wall thickness of 2.5 mm expanded beads having a bulk density of about 50 g/1 are used whilst for a wall thickness of 1.5 mm expanded beads having a bulk density of over 100 g/1 are used, 90% of the beads then having a diameter of only between about 0.3 and 0.5 mm so that at least three beads can still be present adjacent each other to fill the wall cross-section without the beads jamming against the wall on introduction into the mold.
This method of simply reducing the preexpansion with small wall thickness of the pattern and thus producing correspondingly smaller expanded beads cannot be adopted for making disposable patterns for full mold casting. For an essential requirement of full mold casting is that the foamed material rapidly gasifies practically without residue when the melt enters and this requires a low density of the foamed material below 20 kg/m.sup.3, or even better below 18 kg/m.sup.3. This corresponds to a bulk density of the expanded beads of also about 15 to 20 k/1 because for the present purposes the foam mold must be completely filled with expanded beads and the remaining expansion in the foam mold cannot lead to any further appreciable reduction of the density. Thus, it is not possible in the present field to employ a small degree of prefoaming to obtain expanded beads of small diameter.
This would leave the alternative of using a particularly fine-bead screen fraction of the preliminary product, prefoaming the latter with the desired high degree of preexpansion to obtain the low density, and using this product because for the same degree of prefoaming the expanded beads produced have of course a diameter which is the smaller the finer the beads of the preliminary product. However, unfortunately there are limits to this approach as well because for various reasons a maximum content of foaming agent of the order of magnitude of 6% by weight can be introduced into the preliminary product, propane and/or isopropane, butane or the like being using as foaming agent. However, part of the foaming agent escapes from the preliminary product due to aging, mechanical action and the like. Since however a particularly fine-bead screen fraction has a relatively greater surface area than a coarse fraction, it is precisely with such a fine-bead fraction that the mechanical action in the screening operation is greatest and the greatest surface area is available for the foaming agent to escape during storage so that as a rule fine-bead fractions retain less expandability than coarse-bead preliminary products. Since however on the other hand for the foaming operation and the subsequent final foaming in the mold a certain minimum foaming agent content is essential, this puts a limit on the fineness of the beads of the preliminary product which, since for the present purposes the finest possible beads are used in any case, does not make it possible to appreciably get below a bead size of the expanded material of about 2 mm. This in turn gives the minimum wall thickness of the casting mentioned at the beginning of about 5 to 7 mm if satisfactory filling is to be ensured in series production. When using still finer beads as preliminary product the proportion of foaming agent used for the preexpansion to the desired low density is so high that for the following final expansion in the mold the remaining foaming agent pressure is no longer adequate for agglomeration or "fusion" of the beads to form a good surface.
The problem underlying the invention is to provide a method of making disposable casting patterns consisting of expanded foamed material beads for full mold casting, preferably for the production of series castings, which permits a good filling even of small mold cross-sections of the foam mold in series casting and in the final foaming in the mold a good agglomeration of the expanded beads and formation of a good surface of the foamed plastic pattern.